1. Field of the Invention
The invention relates to an offset printing machine, in particular a web-fed rotary offset printing machine, with one or more printing units arranged one behind the other in the direction of transport of the print carrier web for printing the web on both sides with one or more colors. Each printing unit includes a plate cylinder and a blanket cylinder on each side of the web. Each plate cylinder carries a finite or sleeve-shaped printing plate, and each blanket cylinder carries a finite or sleeve-shaped rubber blanket arranged for transfer of ink from the corresponding printing plate. The invention further relates to a method for operating such an offset printing machine.
2. Description of the Related Art
It is already known from U.S. Pat. No. 4,953,461 to change the circumferential speed between the plate cylinder and rubber-blanket cylinder in order to eliminate the so-called ink build-up (piling) in offset printing machines, so that, whenever the plate cylinder rotates during the printing operation, the image is transferred from the printing plate onto the rubber blanket at a different location, these changes being so slight that they have either no effect or only an acceptable or insignificant effect on the printed product. Furthermore, it is known from U.S. Pat. No. 5,245,923, likewise for the purpose of avoiding an ink build-up, to cause the rubber sleeve arranged on the rubber-blanket cylinder to creep, that is to say xe2x80x9cslipxe2x80x9d, on the rubber-blanket cylinder, so that the printing plate or printing sleeve on the plate cylinder likewise transfers the inked printing image on the surface of the rubber blanket at different locations, so that an accumulation of ink during printing is avoided. On the one hand, it is scarcely possible for a controlled creeping of the rubber-blanket sleeve on the rubber-blanket cylinder to be carried out and, on the other hand, a variation in the creep or in the extent of the creep on the rubber-blanket cylinder is not possible during printing.
The known measures described above therefore eliminate only the adverse effect of the so-called ink build-up on the rubber blanket on the printing image applied by the latter. As a rule, the quality of the image to be printed is impaired unacceptably only when a considerable ink amount has xe2x80x9cbuilt upxe2x80x9d, that is to say accumulated. In this case, an interruption in the printing operation is also often advisable, in order to wash the rubber blanket having the ink build-up or all the rubber-blanket cylinders involved in printing. If an appropriately large number of printed copies have then already been printed, this does not have an excessively disruptive or adverse effect.
It is also generally known, however, that, in offset printing machines, in particular web-fed rotary offset printing machines operating on the blanket-to-blanket principle, that is to say so-called illustration printing machines with horizontal web guidance, particularly during the printing of high-grade papers, that is to say, for example, coated or calendered papers, even after only a few thousand printed copies a kind of rubber-blanket ink build-up occurs, which makes it necessary to wash the rubber blanket even after such a short operating period. Various measures for eliminating this effect have not hitherto been entirely successful.
In particular, so-called light screens, that is to say screens with relatively few or small screen dots per unit area, react particularly sensitively to this specific rubber-blanket ink build-up, to be precise on the rear side of the sheet or web, that is to say the ink transfer behavior changes in these screen surfaces as a result of a build-up on the rubber blanket on the other side of the sheet or web. In other words, for example when ink builds up on the rubber-blanket cylinder arranged below the horizontally guided print carrier web, on the top side of the web or in the printing image applied to the latter by the upper rubber-blanket cylinder the quality of the printing image is influenced adversely, this also being known as reverse pilingxe2x80x9d.
As an initial finding for the invention described below, it was established, for the first time, that the above-described quality fault in the printing image becomes noticeable at that point, that is to say on the rubber-blanket cylinder, where the web last comes loose from the rubber-blanket cylinder. As is generally known, normally, the rubber-blanket and plate cylinders lying in one plane are positioned slightly obliquely, so as to result in a partial looping of the web around one of the rubber-blanket cylinders. Alternatively, this partial looping of the print carrier web around one of the cylinders may also be achieved by the rubber-blanket cylinders located one above the other being offset slightly in a suitable way.
Proceeding from this, the object of the present invention, in offset printing machines, in particular in web-fed rotary offset printing machines operating on the blanket-to-blanket principle, is to provide a way to delay or prevent the disturbing premature ink build-up.
According to the invention, the rubber blanket cannot undergo any movement relative to the rubber blanket cylinder. A lateral register is arranged to move each plate cylinder axially with respect to the corresponding blanket cylinder, and a circumferential register is arranged to move each said plate cylinder circumferentially with respect to the corresponding blanket cylinder.
Either the lateral registers or the circumferential registers, or both, are adjustable by predetermined values either at predetermined time intervals or continuously. The corresponding plate cylinders are displaceable in either the axial or circumferential directions, or both, by an amount corresponding to the corresponding predetermined values.
In the method according to the invention, commands are fed to either of the registers, or both, for at least one of lateral and circumferential displacement of the corresponding plate cylinder by predetermined amounts at predetermined time intervals or continuously.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.